This invention relates to a printing paste for marking non-weldable areas in the manufacture of so-called ducted plates, in which at least two metal plates consisting preferably of aluminum or aluminum alloys are joined by sealed assembly rolling.
In the above-described manufacture of so-called ducted plates, a sealed assembly rolling step, which is conventional also for cladding is employed in which at least two superimposed plates are preheated, if desired and are then firmly joined by the rolling pressure, whereby the thickness of the plates can be more or less decreased in conjunction with a corresponding stretching of the plates in length. Whereas the plates are normally joined throughout their interface by sealed assembly rolling, it is desired in the manufacture of so-called ducted plates to leave certain areas unwelded, in a predetermined pattern, so that the assembly can be subsequently expanded by the supply of a pressure fluid to form a duct or cavity system for receiving or transferring a flowable heat-carrying medium. Such ducted plates have found wide use as evaporators in refrigerators, freezers or the like.
Whereas graphite in various dispersions has been used to avoid a welding in predetermined areas, it has been found that difficulties often arise when graphite is applied by screen process printing, because the contours of the "picture" are not sharp enough since the graphite paste cannot be properly squeezed through the screen and/or because it does not sufficiently adhere to the marked plate and is peeled off in part as the screen is lifted from the plate. It has also been found that graphite cannot be completely removed from the expanded duct system and that the residual graphite in conjunction with some heat-carrying media may give rise to corrosion.
For this reason it has already been proposed to provide a graphite-free, weld-resist composition which consists of an aqueous dispersion of 5 to 60% by weight of titanium dioxide, 0.5 to 5% by weight of bentonite, 0.10 to 4% by weight of magnesium montmorillonite and, if desired, 0.1 to 10% by weight of silica, and the balance of water (German Patent Specification No. 1,508,342). Whereas this mixture separates easily from the screen, difficulties arising in its use residue in that it does not completely cover the areas to be printed and that it contracts, owing to a high surface tension, so that the plates were welded together also in parts of the areas which are not to be welded. In many cases, the formation of the ducts exactly in accordance with a predetermined pattern is decisive for the function of equipment in which the ducted plate is used. For this reason, the known weld-resist composition cannot be used with satisfactory results where the dimensional accuracy of the duct pattern need meet particularly stringent requirements.
For a long time, the applicant has marked non-weldable areas in the manufacture of ducted plates with a graphite-free printing paste consisting of a dispersion of 25 to 35 grams of a water-soluble alginate such as ammonium alginate, 50 to 70 grams of Aerosil and 2 to 4 ml of highly concentrated ammonia solution in 1000 ml of water. Whereas that printing paste has satisfactory printing properties, a dry coating formed by it does not satisfactorily separate the component plates so that very high pressures were required to inflate the ducts. Besides, that dispersion has to be used with very great care because a relatively great reduction in thickness by the rolling step (by which the thickness may be decreased down to 1/5 of the initial thickness) accompanied by corresponding stretching of the plates in length may so strongly reduce the separation of the component plates in certain areas that the ducts can no longer be inflated at all.